U.S. patent number 8,072,220 [Application Number 12/078,069] was granted by the patent office on 2011-12-06 for positioning, detection and communication system and method.
This patent grant is currently assigned to Raytheon UTD Inc.. Invention is credited to Steven A. Cotten, Benjamin P. Dolgin, Luis B. Giraldo, John T. Ishibashi, Kenneth D. Kuck, Craig E. Matter, Michael Shore.
United States Patent |
8,072,220 |
Dolgin , et al. |
December 6, 2011 |
Positioning, detection and communication system and method
Abstract
A positioning, communication, and detection system designed to
provide a three dimensional location of an object, navigation
tools, and bidirectional surface-to-subsurface communications, and
methods of using the system. The system can include one or multiple
transmitters comprising electromagnetic beacons, software defined
radio receivers with an associated processing unit and data
acquisition system, and magnetic antennas. The system may use
theoretical calculations, scale model testing, signal processing,
and sensor data.
Inventors: |
Dolgin; Benjamin P.
(Alexandria, VA), Shore; Michael (Chapel Hill, NC),
Cotten; Steven A. (Dumfries, VA), Matter; Craig E.
(Ashburn, VA), Kuck; Kenneth D. (Fairfax, VA), Giraldo;
Luis B. (Fairfax, VA), Ishibashi; John T. (Burke,
VA) |
Assignee: |
Raytheon UTD Inc. (Springfield,
VA)
|
Family
ID: |
41417028 |
Appl.
No.: |
12/078,069 |
Filed: |
March 26, 2008 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20090009410 A1 |
Jan 8, 2009 |
|
Related U.S. Patent Documents
|
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
11640337 |
Dec 18, 2006 |
|
|
|
|
60750787 |
Dec 16, 2005 |
|
|
|
|
Current U.S.
Class: |
324/329;
324/228 |
Current CPC
Class: |
G01C
21/165 (20130101) |
Current International
Class: |
G01V
3/10 (20060101); G01N 27/72 (20060101) |
Field of
Search: |
;324/228,329 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
International Search Report and Written Opinion of the
International Searching Authority, Sep. 2010. cited by
other.
|
Primary Examiner: Ledynh; Bot
Attorney, Agent or Firm: Dickstein Shapiro LLP
Government Interests
GOVERNMENT RIGHTS
Part of the work performed during development of this invention
utilized U.S. Government funds. The U.S. Government may have
certain rights in the invention.
Parent Case Text
This application is a continuation in part of U.S. patent
application Ser. No. 11/640,337, filed on Dec. 18, 2006, which
claims the benefit of U.S. Provisional Patent Application Ser. No.
60/750,787, filed on Dec. 16, 2005, the entire disclosure of each
is incorporated herein by reference.
Claims
What is claimed as new and desired to be protected by Letters
Patent of the United States is:
1. A navigation system comprising: at least one transmitter
comprising at least two magnetic dipoles, said transmitter being
configured to generate a magnetic field; and a receiver comprising
a magnetometer configured to receive input from said at least one
transmitter; and wherein the transmitter is configured to change
the magnetic dipoles' respective amplitudes according to one or
more predetermined patterns, thereby producing associated signals;
wherein the transmitter is configured to change the amplitude of a
first magnetic dipole at a first frequency and the amplitude of a
second magnetic dipole at a second frequency, where the first
frequency is different from the second frequency; wherein the
magnetic dipoles are configured to rotate around respective axes at
a constant rate; wherein the associated signals are configured to
rotate in a fixed plane; and wherein the receiver is configured to
determine a bearing of the transmitter based on an orientation of
the fixed plane.
2. The navigation system of claim 1, wherein the receiver is
configured to identify each transmitter based upon differences
between their associated signals.
3. A navigation system comprising: at least one transmitter
comprising at least two magnetic dipoles, said transmitter being
configured to generate a magnetic field; and a receiver comprising
a magnetometer configured to receive input from said at least one
transmitter; and wherein the transmitter is configured to change
the magnetic dipoles' respective amplitudes according to one or
more predetermined patterns, thereby producing associated signals;
wherein the transmitter is configured to change the amplitude of a
first magnetic dipole at a first frequency and the amplitude of a
second magnetic dipole at a second frequency, where the first
frequency is different from the second frequency; wherein the
magnetic dipoles are configured to rotate around respective axes at
a constant rate; and wherein the receiver is configured to
determine the line of bearing to the transmitter based on the
difference in orientation between at least the first magnetic
dipole and the second magnetic dipole.
4. The navigation system of claim 3, wherein the receiver is
configured to determine a distance to the transmitter based on
amplitude signals of the magnetic dipoles.
5. A navigation system comprising: at least one transmitter
comprising at least two magnetic dipoles, said transmitter being
configured to generate a magnetic field; and a receiver comprising
a magnetometer configured to receive input from said at least one
transmitter; and wherein the transmitter is configured to change
the magnetic dipoles' orientations at different respective
frequencies; and wherein the transmitter further comprises a first
clock, the receiver further comprises a second clock, wherein the
first and second clocks are synchronized for use in signal
detection.
6. The navigation system of claim 5, wherein the receiver
synchronizes the second clock with the first clock based on a
difference in the magnetic dipoles' orientations.
7. The navigation system of claim 5, wherein the magnetic dipoles
are spinning dipoles.
8. The navigation system of claim 5, wherein the magnetic dipoles
share a center of rotation.
9. The navigation system of claim 5, wherein the transmitter
further comprises at least two non-coaxial magnetic coils for
generating the magnetic field.
10. The navigation system of claim 5, wherein the transmitter
further comprises at least three non-coaxial magnetic coils for
generating the magnetic field.
11. The navigation system of claim 10 wherein at least one of the
magnetic coils include a magnetic core.
12. The navigation system of claim 11 wherein the at least two
magnetic coils share the same magnetic core.
13. The navigation system of claim 5, further comprising a device
for determining a line of bearing of the receiver relative to the
transmitter.
14. A navigation system, comprising: a transmitter comprising at
least two rotating, co-located magnetic dipoles, said co-located
magnetic dipoles sharing an axis of rotation; and a receiver
comprising a magnetometer; and wherein said receiver is configured
to use a signal produced by the two rotating, co-located magnetic
dipoles as a clock signal.
15. The navigation system of claim 14, wherein said at least two
co-located magnetic dipoles are generated by two or more magnetic
coils.
16. A navigation system, comprising: a transmitter comprising at
least two rotating, co-located magnetic dipoles, said co-located
magnetic dipoles sharing an axis of rotation; and a receiver
comprising a magnetometer; and wherein the receiver is configured
to use a signal produced by the two rotating, co-located magnetic
dipoles to obtain a line of bearing relative to the transmitter.
Description
FIELD OF THE INVENTION
The disclosed embodiments relate generally to methods and devices
pertaining to a positioning, detection and communication
system.
BACKGROUND
Geological mapping and geophysical surveying on the earth's surface
are mature sciences with a history of technology enhancements that
improved the fidelity of understanding of the Earth, above and
beneath the surface. Yet when conventional techniques are employed
in an underground environment, geo-location has proven a challenge
that drives concepts of operations to bootstrap techniques to
geo-locate instrumentation and geological contacts and can actually
limit the effectiveness of employed technologies.
Conventional mapping and survey systems, such as a Global
Positioning System (GPS), determine the location of objects using
satellite signals. However, a longstanding problem exists with
determining location of personnel and equipment within, for
example, underground facilities without the use of surveying. To
date, this problem has not been resolved because of the difficulty
of signaling/communicating between the Earth's surface and
underground facilities/caverns/mines and the complexity of
electromagnetic propagation within the Earth.
Lower fidelity very low frequency systems are currently in
development in Europe to support communications for cave rescue
operations. The systems only obtain a shallow depth position when
the communication system is used underground. These communications
systems are effective up to 600 m and occasionally up to 1,200 m.
The systems are also used to locate underground transmitters at
comparable depths. In controlled experiments, they have achieved an
accuracy of 2% in horizontal position and only 5% in depth.
The typical means of providing time base synchronization between a
transmitter and receiver used for navigation purposes has been to
either (1) provide a uniform time radio reference signal from an
independent source (GPS or VLF signal) or (2) provide each
transmitter and receiver with its own highly accurate and stable
timing mechanism which are then mutually synchronized at the
beginning of the period of interest. In underground environments,
GPS and VLF signals are either unavailable or unreliable. Providing
each device with its own stable time base may be expensive,
cumbersome, and wasteful of limited available power supply.
Normal radio frequency wireless communications to/from a
sub-surface receiver by a surface transmitter have been unavailable
due to the electrical properties of ground, soil and rock.
Communications beyond a depth of 100 meters is particularly
difficult. A system that provides wireless contact between
subterranean and surface locations will be desirable. Particularly
such a system that could provide accurate positioning, detection
and communications between the Earth's surface and sub-surface.
SUMMARY
The system provides a means for location determination in the
underground, determination of subterranean masses, and
surface-to-subsurface communications. This development is made
possible through the assembly of sensor technologies and processing
capabilities that are currently evolving at the state-of-the-art in
several diverse arenas.
The system can provide individuals and equipment moving within a
space, either above or below ground, the capability to know their
location in three dimensions. The system identifies the location of
an object by applying theoretical calculations, and novel
technology demonstrations including state-of-the-art signal
processing, fusion of multiple sensor data, and unique concepts of
operation, which include magnetic beacons and special Software
Defined Radio (SDR) receivers to determine the location of an
object, above or below ground. A back channel communications
capability is provided.
An exemplary embodiment of the system uses multiple transmitters on
the surface, in the vicinity of an underground space, to provide
magnetic beacons. The signal processing can be supplemented with
distant signals of opportunity, both cooperative and uncooperative.
The SDR receiver carried underground can measure angles between
various transmitters. The surface transmitter locations can be
determined when deployed and the magnetic radiation field can be
calculated so that the underground receiver location can be
determined.
An inertial guidance unit can be included as a part of the
processing unit to provide a stable reference as a stop-gap
navigation capability. In addition to the SDR receiver and inertial
guidance unit, disclosed embodiments can employ accelerometers/tilt
measurement devices, magnetic compass, microbarograph, ranging on
the back channel communications system, and automated
pacing/velocity devices.
Multiple magnetic dipoles spinning around an axis can be used to
provide measurements allowing position calculations without
requiring a particular receiver orientation. A magnetic core
antenna can be provided to increased transmitter range so as to
allow for surface-to-subsurface bidirectional communications.
These and other features of the disclosed embodiments will be
better understood based on a reading of the Detailed Description
below, in view of the figures, which are a part of this
specification.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a positioning system architecture.
FIG. 2 shows a block diagram of a software defined radio receiver
according to an embodiment.
FIG. 3 shows a transceiver according to an embodiment.
FIG. 4 shows a block diagram of a magnetic beacon transmitter.
FIG. 5A shows a spherical core antenna and a horizontal loop
antenna.
FIG. 5B shows a rod core antenna.
FIG. 6 illustrates an analysis of a positioning system according to
an embodiment.
FIG. 7 shows an error analysis for the positioning system according
to an embodiment.
FIG. 8 shows transmitter coverage upon deployment in accordance
with an embodiment.
FIG. 9 shows a method of subsurface scanning in accordance with an
embodiment.
FIGS. 10-11c show variations of a magnetic dipole.
FIG. 12 shows a field line of a magnetic beacon in polar
coordinates.
FIG. 13 shows a variation of a magnetic dipole.
FIG. 14 shows a system in accordance with an embodiment.
FIG. 15 shows the interaction between a transmitter and
receiver.
FIG. 16 is a chart showing the relationship between effective
magnetic moment and a coreless coil magnetic moment.
DETAILED DESCRIPTION
In the following detailed description, reference is made to the
accompanying drawings, which form a part hereof and show by way of
illustration specific embodiments in which disclosed embodiments
may be practiced. These embodiments are described in sufficient
detail to enable those skilled in the art to practice the disclosed
embodiments, and it is to be understood that other embodiments may
be utilized, and that structural, logical, and other changes may be
made without departing from the spirit and scope of the presently
disclosed embodiments.
An exemplary positioning system 10 is shown in FIG. 1. The
positioning system 10 is comprised of a number of components, which
can include transmitters 12 (as used herein, the terms
"transmitter" and "beacon" are interchangeable) and a SDR unit 14
("receiver"). Additional signals of opportunity 13, such as from
other transmitters in the very low/low/medium frequency range and
AM radio signals, can also be exploited as additional signal
sources, as will be explained further below.
FIG. 2 shows a block diagram of a receiver 14. The receiver 14
comprises a sensitive, three-component magnetic receiver capable of
accurately detecting the magnetic field vectors emanating from the
transmitters 12, a processing unit 15, a power source 42, a GPS
receiver 17, an inertial guidance unit 19, a magnetic antenna 31, a
dipole antenna 33, a signal processor 43, a VHF transceiver 45, a
land navigator system 47, and additional secondary sensors 30
(e.g., magnetic compass, accelerometers, tiltmeters,
microbarometer).
The processing unit 15 processes data received by the three-channel
VLF receiver 35, the dipole antenna 33, and secondary sensors 30 to
provide a three-dimensional location of the receiver 14, either
below or above ground. The inclusion of the GPS receiver 17 allows
the receiver 14 to interface with an existing GPS-based land
navigation unit and provide full integration with surface
geographic information systems and databases. The processing unit
15 output 24 may be accordingly configured so that existing land
navigation options for display and user interface are preserved and
underground locations obtained from the positioning system 10
smoothly transition from GPS locations determined during times that
the receiver 14 is above the Earth's surfaces 5.
The processor 15 can also store reference locations of each of the
transmitters 12, as well as the surveyed information about the
signals of opportunity 13. These data can be used in estimating the
current position of the user. GPS locations of the entry points are
used to provide the "truth" for the starting positions. The outputs
from the microbarometer (part of secondary sensors 30) of the
receiver 14 can also be used to provide incremental update and
error correction for elevation estimates. Using this data, the
computed location can be continually updated on the display output
24.
Magnetic fields induced by the transmitters 12 are detected by the
receiver's 14 magnetic antenna 31. A preferred magnetic antenna 31
for use with the receiver 14 is the Raytheon Cube sensor, a
triaxial air coil magnetic receiver that is currently one of the
most sensitive instruments available with a noise floor at 10 kHz
of 0.6 ftesla/sqrt Hz for the 12-inch antenna and 5 ftesla/sqrt Hz
for the 6-inch antenna. The processing unit 15 operates a three
component VLF receiver 35 and signal processor 43 to calculate the
azimuth and inclination of vector magnetic fields induced by the
transmitters 12. Using the known locations of the transmitters 12
and azimuths to distant transmitters 12, the processing unit 15
determines the receiver 14 location on a continuous basis as the
receiver 14 is moved within the underground space.
Motion induced noise from movement of the receiver 14 can
potentially decrease the accuracy of the system 10 and preferably
should be reduced below the noise floor of the system 10 for
typical user motions. The frequency of operation can mitigate
unwanted noise, as the components of user noise induced at the
operational frequency are small. Taking this into consideration,
the receiver 14 is designed such that motion of components in the
very low frequency range of interest (preferably below 10 kHz) are
minimal. It should be understood that the method of taking into
account such design consideration may be embodied in various ways
according to the particular constraints of the receiver 14, which
may be physical, electrical or aesthetic. For example and without
limitation, the antennae 31, 33 can be encased in damping
materials, e.g., foam, that substantially attenuate motion
components in this range. This can be done with relatively small
volumes of damping material. Furthermore, sufficient dynamic range
on the antenna 31 outputs can be provided such that out of band
motion induced noise (primarily in the extremely low frequency
range) do not overload the electronics. Tilt sensors (part of other
sensors 30) may be included on the antennae 31, 33 to measure
antenna motion. Micro-electro mechanical sensor-based, solid state
tilt sensors can be used for this purpose. With suitable motion
information, adaptive filtering can be used to further reduce the
effects of motion on the antenna 31, 33. Complete Faraday shielding
of the antenna 31, 33 can be helpful to reduce potential
interference from outside interferers.
Navigation in underground environments is possible using an
embodiment of the system 10 having transmitters 12 with two or more
co-located magnetic dipoles with known magnetic properties (e.g.,
frequency, amplitude, and dipole orientation) or rotating dipoles
(dipoles excited at a given frequency with the dipole direction
rotating at a known speed around a known axis), as shown in FIGS.
11b and 11c. The rotating dipoles are preferred and comprise at
least two dipole wires 2 with a modulated signal such that the
dipole magnetic moment rotates around an axis of rotation 6
producing an associated amplitude signal that can be detected by
receiver 14. This approach permits potential use of a smaller
number of transmitters 12, which would also provide a more robust
navigation solution. Previous navigation schemes required at least
three operating beacons 12. This embodiment permits navigation from
a single VLF navigation beacon 12 comprising two or more co-located
transmitting magnetic dipoles.
If magnetic antenna 31 and VLF receiver 35 calibration is known and
magnetometer and transmitter 12 are synchronized, exact position of
the magnetometer can be obtained from a single beacon 12 in an
empty space. If the rotating dipole rotates in all three dimensions
of a beacon 12, then the bearing in global coordinate system can be
obtained using a single transmitter 12.
Navigation or location solutions for the rotating dipole embodiment
can also extend to co-located dipole embodiments. As shown in FIG.
10, a co-located dipole beacon 12 is a beacon 12 that comprises two
or more oscillating magnetic dipoles that are separately actuated.
The dipoles are positioned in such a way that their centers 3 are
in the same spot. The orientations of magnetic moments 4 for each
dipole are different. A cube with three coils wrapped around its
perpendicular faces is an example of a co-located dipole. A sphere
with several coils is another. The transmitter 12 can be based on a
ferromagnetic core 44 (sphere, cube, etc; FIGS. 5a and. 5b) or it
can be coreless.
FIG. 10 is an example of a coreless co-located dipole beacon 12
based on two coils 2. The figure shows two co-located dipoles. Two
wire coils 2 carry currents from two separate power supplies. The
coils 2 are stationary in space, but current in each coil 2 is
modulated differently. For example, one coil 2 is actuated at
frequency f1 while another at frequency f2 resulting in an
associated amplitude signal that can be detected by receiver
14.
A spinning dipole beacon 12, as shown in FIGS. 11a to 11bc, is a
magnetic dipole that is spinning around an axis 6 in space. An
embodiment includes a transmitter 12 with the axis of rotation 6
perpendicular to the orientation of the resulting magnetic dipole
rotating with constant angular velocity. FIG. 11a shows a magnetic
beacon comprising of a magnetic dipole that is being rotated around
an axis 6 perpendicular to its magnetic vector 4 (dipole magnetic
moment). FIG. 11b shows a magnetic beacon 12 with the field
equivalent to that in FIG. 11a; the two wire coils 2 are
perpendicular to each other. The current source is modulated by a
signal equal to the sine and cosine of the rotation phase. FIG. 11c
shows a beacon 12 capable of 3D rotation of the effective magnetic
dipole (three co-located dipoles).
A spinning dipole beacon does not need to have any moving parts.
For example, a beacon described in FIG. 11b with two magnetic coils
2 perpendicular to each other will produce the same field if the
current source actuating the two orthogonal coils 2 in FIG. 11b is
producing currents defined by the following Equation 1:
.times..function..phi..times..function..phi..times. ##EQU00001##
where I.sub.Green and I.sub.Blue are respective currents through
the two coils 2 and I.sub.Rotation is the current through rotating
coil, and .phi..sub.Rotation is the angle of rotation of the
rotating coil. Similar formulas can be derived for beacons
comprising coils that are not orthogonal.
In a constant angular velocity case, the equation defining such
currents can be shown by Equation 2, as follows:
.times..function..omega..times..times..times..function..OMEGA..times..tim-
es..PHI..times..function..omega..OMEGA..times..PHI..function..omega..OMEGA-
..times..PHI..times..function..omega..times..times..times..function..OMEGA-
..times..times..PHI..times..function..omega..OMEGA..times..PHI..function..-
omega..OMEGA..times..PHI..times. ##EQU00002##
In other words, a rotating dipole is just a special case of a
general co-located dipole. Full 3D rotation of the dipole is an
equivalent of a 3 co-located dipoles (FIG. 11c). In an equivalent
formulation, the magnetic moment of the beacon is described by the
following Equation 3:
.times..times..function..OMEGA..times..times..PHI..times..times..function-
..OMEGA..times..times..PHI..times. ##EQU00003## where M=cos
.omega.t is the dipole value, .OMEGA. and .PHI. are rotation
frequency and phase, and .omega. is the beacon carrier frequency.
For simplicity, the phase of the beacon carrier frequency signal is
set to 0.
Co-located dipoles permit line of bearing (LOB) to be determined by
a receiver 14 with an unknown orientation. To solve for LOB one
determines 5 variables: 2 angles to the position of the receiver 14
in the magnetic dipole (beacon) coordinate system and 3 angles that
determine orientation and position of the beacon in the receiver 14
coordinate system. Theoretically, the distance can be determined as
well. The total geolocation requires measurement of a sixth
variable: the distance between the beacon and the receiver 14.
The magnetic field measurements produce three measurements per
magnetic dipole in a collocated transmitter 12 beacon. Thus, any
collocated beacon permits LOB determination in the receiver 14
coordinates.
Where a magnetic beacon is located in the origin of a Global
Coordinate system (GCS) and the co-located beacon is a spinning
beacon with the dipole rotating around z axis 6 in GCS, the value
of the magnetic vector in GCS is described by Equation 3, above.
The magnetic field (B) of the dipole is determined by Equation 4,
as follows:
.mu..times..pi..times..times..times. ##EQU00004##
Thus, the value of magnetic field at a point r in the GCC,
where:
##EQU00005## is expressed by Equation 5, as follows:
.mu..times..times..pi..times..times.
.times..times..times..times..times..times..times..times..OMEGA..times..ti-
mes..times..THETA..times..times..times..times..times..times..times..times.-
.times..times..OMEGA..times..times..THETA. ##EQU00006##
FIG. 12 shows a magnetic beacon in polar coordinates. The beacon is
located in the origin of the X,Y,Z coordinate system. The receiver
14 unit is at the point of origin of vector B. The dipole magnetic
moment 4 vector M denotes instantaneous orientation of the magnetic
moment of the beacon at a particular point in time. The
instantaneous magnetic field line 32 for the current position of
the spinning beacon is shown. The magnetic field line 32 intersects
the magnetometer position. Magnetic moment 4 is excited by a
magnetic coil, e.g., 2, operating at a fixed frequency below 10 kHz
while it is simultaneously rotating around Z axis 6 at several
dozen rpm. In a polar coordinate system defined by the receiver 14
and the center 3 of the dipole, the values of magnetic moment 4 and
the distance are defined by Equation 6, as follows:
.times..times..function..OMEGA..times..times..PHI..phi..times..times..tim-
es.
.times..times..function..OMEGA..times..times..PHI..phi..times..times..-
function..OMEGA..times..times..PHI..phi..times..times..times.
.times..times..times..times. .times. ##EQU00007## Where e.sub.i,
refers to a unitary vector in corresponding direction. Thus, the
component magnitudes of the magnetic field B are defined by
Equation 7, as follows:
.mu..times..times..pi..times..times..function..OMEGA..times..times..PHI..-
phi..times..times.
.phi..mu..times..times..pi..times..times..function..OMEGA..times..times..-
PHI..phi.
.mu..times..times..pi..times..times..function..OMEGA..times..tim-
es..PHI..phi..times..times. .times. ##EQU00008##
The important feature of the Equation 7, above, is the fact that it
separates the radial (r), azimuth (.phi.), and incline (.theta.)
dependences of the magnetic field. The square of the magnetic field
value can be determined from Equation 8, as follows:
.times..mu..times..times..pi..times..times..times..function..OMEGA..times-
..times..PHI..phi..times.
.times..mu..times..times..pi..times..times..times. .times.
.function..OMEGA..times..times..PHI..phi..times..mu..times..times..pi..ti-
mes..times..times..times..times. .times..times..times.
.function..OMEGA..times..times..PHI..phi..times. ##EQU00009##
Note that the value of |B|.sup.2 is independent of the actual
orientation of the receiver 14. However, if the time dependence of
|B|.sup.2 is known, it provides enough equations to solve for
distance (r), azimuth (.phi.), and incline (.theta.) in the
GCS.
LOB Navigation in the receiver 14/Local Coordinate System (LCS) can
be performed using this embodiment. The receiver 14 measures the
instantaneous values of magnetic field B.sub.x, B.sub.y, B.sub.z.
To determine the LOB of the beacon 12 one has to find an
orientation in the LCS in which the time dependence of the
corresponding magnetic vector components would satisfy Equation 7.
To find this orientation, one can remove carrier frequency by
fitting the value of the magnetic filed into cos .omega.t and sin
.omega.t and shifting the frequencies down by the value of .omega..
As follows for this embodiment, the described values of magnetic
field are algebraic values of the modulation. The values of
magnetic field B.sub.x, B.sub.y, B.sub.z are fit into cos .OMEGA.t
and sin .OMEGA.t so that the following Equation 9 holds: {right
arrow over (B)}.sub.x=a.sub.x cos(.OMEGA.t+.PHI.)+b.sub.x
sin(.OMEGA.t+.PHI.) {right arrow over (B)}.sub.y=a.sub.y
cos(.OMEGA.t+.PHI.)+b.sub.y sin(.OMEGA.t+.PHI.) {right arrow over
(B)}.sub.z=a.sub.z cos(.OMEGA.t+.PHI.)+b.sub.z sin(.OMEGA.t+.PHI.)
(Eq. 9) The defined vectors are:
##EQU00010## ##EQU00010.2##
Angles .alpha. and .beta. are found so as to define the rotation of
the magnetic field detector using Equation 10, as follows:
.times..times..alpha..times..times..alpha..times..times..alpha..times..ti-
mes..alpha..times..times..beta..times..times..beta..times..times..beta..ti-
mes..times..beta..times. ##EQU00011## such that the new axis y is
parallel to the plane of magnetic dipole rotation by satisfying
Equation 11, as follows:
.times..times..times. ##EQU00012## and angle .gamma. is determined
by Equation 12, as follows:
.times..times..gamma..times..times..gamma..times..times..gamma..times..ti-
mes..gamma..times. ##EQU00013## so that the new x axis points
toward the transmitter 12 such that Equation 6, as follows, is
satisfied:
.times..times..times. ##EQU00014##
The following Equation 14 is calculated:
.times..times..times..times..times..times..times. ##EQU00015##
points toward the transmitter in the local coordinate system. Once
these two fits are performed, one can calculate direction to
receiver 14 in the beacon coordinate system by noting that the
vector:
.function..times. ##EQU00016## in the Global Coordinate System
points toward the receiver 14. Vector D is not unitary and can be
normalized such that:
.times. ##EQU00017##
Equation 13 holds true after rot.sub.ii is applied. Thus it should
be determined if the fit described in Equation 9 and 10 should be
performed after applying Equation 12 to measured fields of Equation
9 as in the following Equation 17:
.times. ##EQU00018##
To determine the LOB of the receiver 14, one finds an orientation
in the GCS in which the time dependence of the corresponding
magnetic field vector components of the associated amplitude signal
would satisfy Equation 7. To find this orientation, one may remove
carrier frequency by fitting the value of the magnetic field into
cos .omega.t and sin .omega.t and shifting the frequencies down by
the value of .omega.. The instantaneous values of the square
magnetic field strength can be calculated using Equation 18, as
follows: |{right arrow over (B)}|.sup.2=|{right arrow over
(B)}.sub.x|.sup.2+|{right arrow over (B)}.sub.y|.sup.2+|{right
arrow over (B)}.sub.z|.sup.2 (Eq. 18) The value of the magnetic
field strength is fit into cos .OMEGA.t and sin .OMEGA.t, such that
Equation 19, as follows, is satisfied: |{right arrow over
(B)}|.sup.2=c.sub.1 cos(2.OMEGA.+2.PHI.)+c.sub.2
sin(2.OMEGA.+2.PHI.)+c.sub.3 (Eq. 19) The values of azimuth (.phi.)
and incline .theta.in the GCS can be determined using Equation 20,
as follows:
.psi..times..times..times..times. .times..times. .times.
##EQU00019##
The value of c.sub.3 in Equation 19, above, cannot be determined
accurately in a noisy environment, even if the integration is
performed over prolonged time. The value of the ratio of c.sub.1
and c.sub.2 is somewhat less susceptible to noise. In this
environment, a dual spinning beacon, as shown in FIG. 13, can be
introduced such that its magnetic moment 4 (or its associated
signal) is defined by Equation 23, as follows:
.times..function..OMEGA..times..THETA..times..function..OMEGA..times..THE-
TA..times..function..OMEGA..times..THETA..times..function..OMEGA..times..T-
HETA..times. ##EQU00020##
The beacon shown in FIG. 12 is capable of producing a magnetic
moment 4 as described for M by Equation 23 above. Using Equations 8
and 18-20, one can derive the values of the azimuth in coordinate
systems independently defined by M.sub.1 and M.sub.2. The value of
azimuth in the M.sub.2 coordinate system of FIG. 13 is the incline
in the M.sub.1 coordinate system, and vice versa. FIG. 13 shows a
spinning beacon and the related magnetic moments 4, which are
independently spinning in the XY and YZ planes.
To detect the magnetic moments 4 of FIG. 13 separately only one of
the two frequencies (carrier and spinning) that characterize each
magnetic moment need to be different. For example, they may have
the same carrier frequency if rotation frequencies are different.
Conversely, they may have the same spinning frequency if
desired.
It is important to know if the receiver 14 is calibrated and beacon
12 amplitude is known, whether the beacon 12 phase .THETA. is known
and whether the receiver 14 is synchronized, and based on such,
what can be determined. If the receiver 14 is calibrated and beacon
12 amplitude known, and the beacon 12 phase is known and the
receiver 14 synchronized, the exact position of the receiver 14 can
be determined in GCS. If the receiver 14 is not calibrated or
beacon 12 amplitude known, but the beacon 12 phase is known and the
receiver 14 is synchronized, the bearing of the receiver 14 in GCS
can be determined. If the receiver 14 is not calibrated or the
beacon 12 amplitude is not known, and the beacon 12 phase is not
known or the receiver 14 is not synchronized, the bearing of the
receiver 14 in LCS can be determined.
Using a system 10 as shown in FIG. 14, it can be derived that a
beacon 12 with three or more co-located dipoles gives bearings in
GCS and a spinning beacon 12 is not required. In this embodiment,
when a single spinning dipole per beacon 12 is used, the azimuth to
the receiver 14 in the beacon 12 coordinates can be determined.
Three beacons 12 with non-parallel beacon Z axes 6 are used to
triangulate. When multiple (2 or more) spinning dipoles per
transmitter 12 are used, full LOB to the receiver 14 can be
determined. This uses two beacons 12 to triangulate, where one can
be a single spinning dipole. With an actively tuned beacon 12, the
beacon 12 spins around the orientation to the receiver 14 and a
communication channel is used. There, the beacon 12 orientation
tracks the receiver 14 for higher signal-to-noise ratio and full
LOB to the receiver 14 can be determined. As such, two beacons 12
are used to triangulate and lower total energy is used than for a
multiple spinning beacon 12 set up.
In another embodiment, the need to provide the receiver 14 with
independent time-based synchronization with the transmitter 12 for
line of bearing scalar magnetometer navigation using the
co-located, rotating magnetic dipoles is eliminated. In this
embodiment, two magnetic dipoles rotate around the same axis 6 and
it is possible that only two magnetic coils 2 are used. Such an
embodiment can be envisioned by adding a second coil 2 to the
embodiment shown in FIG. 11a so that two dipoles are caused to
rotate around the axis 6, but the phase of signals are at different
beat frequencies. The phase of a signal at the different beat
frequencies generated by the two spinning dipoles is independent of
the position and orientation of the magnetometer and so can be used
as a clock signal. Furthermore, in addition to using the twin
magnetic dipoles for clock synchronization, they may also be used
for navigation.
To measure the angle between real parts of magnetic fields as
described above, each transmitter 12 and receiver 14 should be
provided with highly accurate and stable timing mechanisms (part of
GPS receivers 17, 18 or other sensors 30), which are then mutually
synchronized at the beginning of the period of interest. FIG. 15
shows how the receiver 14 can intercept magnetic field lines 32 of
the signal resulting from the magnetic dipoles of a beacon 12 based
on azimuth 7, incline 8, and magnetic field measurements.
In an environment where conductivity is high, synchronization to a
beat frequency may be used to compensate for errors related to the
time propagation (between the transmitters 12 and the receiver 14)
effects. The magnetic field of two magnetic moments (M) with the
same modulation frequency .omega. rotating around Z axis 6 with
frequencies .OMEGA..sub.1 and .OMEGA..sub.2 is described by the
following Equations 24 and 25:
.times..times..times..times.>.times..function..OMEGA..times..PHI..time-
s..function..OMEGA..times..PHI..times..function..OMEGA..times..PHI..times.-
.function..OMEGA..times..PHI..times..times..omega..times..times..times..ti-
mes..times..times..times.>.mu..times..times..pi..times.>>>>-
.times. ##EQU00021## In latitude/longitude coordinates, the values
of the field are determined by Equation 26, as follows:
>.mu..times..times..times..omega..times..times..times..times..pi..time-
s..times..function..times..times..times..function..OMEGA..times..PHI..phi.-
.times..times.
.times..times..times..function..OMEGA..times..PHI..phi..times..times.
.times..function..OMEGA..times..PHI..phi..times..function..OMEGA..times..-
PHI..phi..times..function..OMEGA..times..PHI..phi..times..times.
.times..function..OMEGA..times..PHI..phi..times..times.
.times..rho..phi. .times. ##EQU00022## Correspondingly, the value
of the square of magnetic field B is determined by Equation 27, as
follows:
.mu..times..times..times..omega..times..times..times..times..pi..times..t-
imes..times..times..times..times..times.
.function..times..OMEGA..times..PHI..phi..times..times..times..times..tim-
es.
.function..times..OMEGA..times..PHI..phi..times..times..times..functio-
n..times..times. .function..times..times.
.times..times..times..times..times..times..times..times..times.
.function..OMEGA..times..PHI..OMEGA..times..PHI..times..times..phi..times-
..times..times..times..function..times..times..times.
.function..OMEGA..times..OMEGA..times..PHI..PHI..times..times..times.
##EQU00023##
Regarding terms 1, 2, and 4 above (Equation 27), each of them, or
all of them together, may be used to determine the azimuth .phi. of
the magnetometer. The first Term (or its equivalent second Term) is
used to determine the azimuth in the case of transmitter 12
comprising a single spinning beacon 12. The fourth term is very
similar to the first two Terms except for it being a beat
frequency. The fifth, Term, the difference beat frequency, is
independent of the azimuth.
The difference beat frequency term may be used for synchronization
as a clock signal. Since the phase value of that Term is
independent of the azimuth, its phase may be used as a clock to
determine the starting time of navigation. In an environment where
conductivity is high, synchronization to a beat frequency may be
used to compensate for time propagation effects since the time
delay of detecting the signal from the fifth Term is very similar
to those for the Terms 1, 2 and 4.
The sum and difference beat frequency Terms may be used to
determine the elevation. The ratio of the amplitudes of the last
two terms depends on elevation only and are expressed by Equation
28, as follows:
.times..times..times..times..times..times..times.
.times..times..times..times..times..times..times. .times.
##EQU00024## The ratio is independent of both azimuth and distance.
Both of these terms can be measured in a noisy environment.
Normally, amplitude ratio is expected to be noisier that the phase
measurement. Unlike the single spinning beacon case, however, none
of these terms are measured at a fixed frequency (2.omega.), but
are equivalents of measuring the difference of signals at two
different frequencies around 2.omega..
In another embodiment, the receiver 14 can also incorporate an
integrated back channel communications path that enables the user
to have elementary communications throughout and outside of the
underground location linked to traditional communications systems
located near the point of entry. As shown in FIG. 3, one embodiment
uses miniature, disposable, easily concealed ad hoc, mesh networked
transceivers 36 for this purpose.
The networking protocol can be configured to allow automatic
network join, relay and update using the receiver 14 and
transceivers 36. A baseline 2.4 GHz radio transceiver 36 measures
less than 21.times.27.times.6 mm including an antenna, or about the
area of a postage stamp. In operation, a user can drop or place
these transceivers 36 as a "bread crumb" trail as he or she moves
along a tunnel or facility. When placed at corners or choke points,
the transceivers 36 are able to communicate several hundred meters
before another one must be placed.
The VHF transceiver 45 (FIG. 2) of the receiver 14 can have a
transceiver 36 embedded in its electronics that communicates with
the "bread crumb" trail. At the entrance to an underground area, a
conventional communications transceiver (not shown) can connect to
a communications channel for the rest of the network supporting the
operation. The transceivers 36 can send and receive data. The
receiver 14 can be configured with methods for an operator to
easily and rapidly enter encoded commands that can be relayed to
and from the communications network. A small, hand held or wearable
personal digital assistant or similar user output device 24 or 16
can be used for this purpose. It is also possible to send and
receive either intermittent or continuous voice communications over
this same network. Users are furthermore able to send their
position to the rest of the operations team. Similarly, users are
able to receive, via the same network, the locations of other users
in a team as they report their positions with other receivers
14.
Referring back to FIG. 1, the transmitters can be surface magnetic
beacons 12 that provide a signal on different frequencies in the
very low/low frequency range. Three to four of these transmitters
12 are usually preferred to support the receiver 14 of the
positioning system 10, such as in its use in underground space.
FIG. 4 shows a block diagram of a transmitter 12. Each transmitter
12 comprises a power supply 16, typically a battery pack capable of
sustaining the system for up to 30 or more hours, extendable with
additional batteries, a processor 25, a Very High Frequency ("VHF")
transmitter 27, a Very Low Frequency ("VLF") transmitter 29, a
dipole antenna 20, and a magnetic loop antenna 21. The transmitter
12 provides an adjustable frequency source detectable by the
receiver 14. The GPS receiver 18 may be used by the processor 25 to
determine the location of the transmitter 12 to within one meter.
The coordinates are transmitted to the receiver 14 as setup data 23
prior to the receiver 14 entering the space of interest, whether
above or below ground. The transmitting antenna 21 may be a simple
coil of wire or a more complex system employing a ferrite core. The
transmitters 12 may be packaged for hand emplacement, for airdrop,
or for being mounted on vehicles.
Referring again to FIG. 1, when the receiver 14 is operated in an
underground space of interest 50, varying amounts of ground, rock,
and soil elements of the surface 5 can be disposed between the
transmitters 12 and the receiver 14. In order to determine the
transmitter 12 output strength required for detection by the
receiver 14 under such circumstances, an operator may assume a 1
Am.sup.2 source and computed the fields at the received location as
a function of frequency (2.pi..omega.), depth (R) and soil
conductivity (.sigma.). For a vertical magnetic dipole at the
Earth's surface 5, the fields are described for the quasi-static
case where the distance from the transmitter 12 to the source is
much less than a wavelength in the conducting medium (e.g., Earth
surface 5). In such a medium, the propagation constant is
determined using Equation 29:
.gamma..sup.2=-.omega..sup.2.mu..epsilon.+j.omega..mu..sigma. (Eq.
29) where .mu. and .epsilon. are the permeability and permittivity
of the conducting medium and .gamma. is the propagation constant.
By definition, the wavelength (.lamda.) in the conducting media is
shown by Equation 30, as follows: 1/|.gamma.|=.lamda. (Eq. 30)
For conditions of:
10.sup.-1<.sigma.<10.sup.-4 mhos
100<R<1000 meters
100<f<10.sup.6 hertz
the principal component of the magnetic field at the walls of a
tunnel at the receiver 14 location is the vertical magnetic field,
determined by Equation 31 as follows:
.times..times..gamma..times..times..pi..times..times..gamma..times..times-
..times. ##EQU00025## where m is the magnetic dipole moment in
Amp-m.sup.2. Making some basic assumptions for typical operating
conditions:
.sigma.=10.sup.-3 mhos
f=10,000 Hz
R=100 and 300 meters
produces the following values for field strength at the receiver
14:
R=100 m, H.sub.z=1.5.times.10.sup.5 fTesla
R=300 m, H.sub.z=1.9.times.10.sup.1 fTesla
Again, the above values assume a 1 A-m.sup.2 transmitter dipole
moment.
The sensitivity of the 6-inch ELF cube baseline antenna for use in
the receiver 14 is 6 ftesla at 10 kHz. Assuming this sensitivity is
tangential (SNR=6 dB), this embodiment can operate at 20 dB SNR,
and band limit noise to 1 Hz to give satisfactory dynamic system
response. Computing the desired transmitter 12 strength shows that
the dipole moments used are 1.6.times.10.sup.-3 Am.sup.2 at 100 m
depth and 0.8 Am.sup.2 at 300 m depth. These are relatively easily
generated signal strengths in the 5 to 10 kHz range. For example,
the battery operated Zonge NT-20 TEM transmitter driving a 1
m.sup.2 loop can readily generate a 25 Am.sup.2 dipole moment. Much
larger moments can be generated by this transmitter using a larger
antenna.
Very low frequency (VLF) magnetic beacons are used to implement the
sub-surface navigation systems disclosed herein. These magnetic
beacons are compact, energy efficient, and powerful, generating a
high magnetic moment with minimum energy. FIG. 5a illustrates an
exemplary dipole antenna 20 and horizontal loop antenna 21 of the
transmitter 12 shown in FIG. 4. The antenna 21 can have the
following characteristics: an air core 44, 100 turns of 37 aluminum
wire, two layers thick, 0.1 m radius and 0.26 m high. An antenna 21
of this configuration would weigh about 3.7 kg and have input
impedance at 10 kHz of 1+j48.OMEGA.. To create a 1 A-m.sup.2 dipole
moment, it could be driven at 0.3 amps at 15 volts or 5 Watts input
power. A power efficient amplifier, Class D, can be used to produce
the drive signal with acceptable levels of harmonic distortion and
at efficiencies of 90%. Thus, for about 6 Watts of battery power,
the transmitter can provide a constant CW transmitter signal.
For a design using 10 D cell LiSO.sub.2 primary battery delivering
175 Watt-hours at 15 volts, the transmitter 12 can operate in
excess of 30 hours. The antenna 21 parameters are not limited to
the above configuration, but may be configured to utilize
optimization to minimize power consumption and produce the largest
transmitted dipole moment as required. The design of the amplifier
electronics is straightforward and will not be discussed further
here.
In order to increase magnetic moment, in another embodiment,
antenna 20 can be constructed using a magnetic core 44 instead of
an air core 44. The magnetic core 44 can boost the effective
magnetic moment with the advantage over an air based core 44 in
that, unlike the number of turns of wire 37, the magnetic core 44
boosts both magnetic moment (M) and inductance (L) by the same
ratio, as shown in FIG. 16. The magnetic permeability can be in the
10-50 range. This can be achieved by using a small diameter ferrite
core 44 or a large diameter foam core 44 with ferrite particles
suspended within. Based on modeling of a single turn magnetic coil
with a 1.001 meter diameter and a magnetic moment of 1 Am.sup.2,
the effective magnetic moment of a coil 37 with a spherical core 44
is expressed by Equation 32 as follows:
.times..times..times..times. ##EQU00026## where M is the magnetic
moment without the core 44 and .mu. is the permeability of magnetic
material. The calculated model follows the graph of FIG. 16.
The magnetic core 44 may be spherical as shown in FIG. 5a or a
cylindrical rod core 44 as the antenna 34 shown in FIG. 5b. A
magnetic core antenna 34 including a magnetic core 44, particularly
a cylindrical rod core 44 can be used to provide two-way
communication between the surface and underground in system 10.
With such an antenna 34 included in the receiver 14 as well as on
the surface, the magnetic moment can be amplified to such an extent
that continuous communications are possible. This allows a user of
the receiver 14 to have surface-to-subsurface, bidirectional,
continuous communications over the system 10.
FIG. 6 shows an elliptically polarized signal 28 and a diagram 26
of received signal power versus antenna orientation. The diagram 26
of the energy distribution shows an elliptically polarized signal
sent by a transmitter 12 and received by a Raytheon Cube used as a
receiver 14. Once signals from beacons 12 are received by the
receiver 14, they can be processed to determine the vector azimuth
of the primary magnetic field from each transmitter 12 as received.
Each channel corresponding to the transmitting frequencies of
antennas 21 on the surface can be processed in this manner to
determine the solid angles between the vector fields of each
transmitter 12. In addition to the signals from the surface
transmitters 12, other signals of opportunity 13 (FIG. 1) such as
navigation beacons, very low frequency communications systems, and
High frequency Active Auroral Research Program (HAARP) can be used
to provide additional information on the location.
The location accuracy of the system 10 is affected by the ability
of the receiver 14 to accurately understand and compensate for
propagation anomalies in the medium between the surface
transmitters 12 and the receiver 14 when the receiver 14 is
underground. Signals of opportunity 13 can sometimes be used to
characterize the medium (e.g., below surface 5). Distant sources of
signals of opportunity 13 can produce essentially uniform fields at
the surface of the region around the operational area. These
uniform fields can provide a source of signals that can be measured
at the receiver 14. By accurately measuring these signals 13, the
effects of inhomogeneities in the medium can be estimated. These
effects can then be used to adjust measured direction of arrival of
signals from the surface transmitters 12 to more accurately predict
receiver 14 location.
In practice, the received signals may not always be as "clean" as
is shown in the example in FIG. 6 because there can be multi-path
energy as well as secondary induced magnetic sources. However, this
apparent clutter can be discriminated from the primary field due to
its signal characteristics and quadrature phase shift. In order to
further discern receiver 14 the location, additional sensors 30
(FIG. 2) as previously mentioned can be employed with the receiver
14 to provide independent information to either directly identify
the location or to assist in weighting the contribution of beacon
12 signals. Additional sensors 30 can include a magnetic compass,
accelerometers/tiltmeters, a microbarograph, ranging between back
channel communications relay cards, and a pedometer for a man-pack
version and an odometer for a vehicle mounted unit.
If, during a period of time in underground operation, no signal is
detected at all, the inertial guidance system 19 (FIG. 2) may
provide updated location information several times per second. In
this manner the receiver 14 may continue operation during times
when transmitters 12 are temporarily out of range or significant
receiver 14 anomalies occur that distort magnetic fields to
negatively impact the calculated location. Another embodiment
permits the use of magnetic fields for localization without
requiring use of an inertial navigation unit to orient the magnetic
field sensor of the receiver 14. If multiple magnetic field sources
from the transmitter 12 of known location and frequency are
available, the magnetic field parameters can be measured
independently of receiver 14 orientation using the angles between
the real parts of the magnetic files created by each transmitter
12. This embodiment is well suited for use with the ferrite core 44
magnetic antennas 20, 21, 34 shown in FIGS. 5a and 5b.
While the inertial guidance system 19 is useful for situations in
which the receiver 14 is out of range of the transmitters 12, it is
less reliable if over-relied upon, occasionally providing erroneous
coordinates due to drifting. It also requires the receiver 14 be
properly oriented, which may be inconvenient at times. The receiver
14 magnetometer can be used as an additional location check during
periods of use when the receiver detects the magnetic field of at
least two transmitters 12. The receiver 14 measures a magnetic
field in its own body coordinate system. Assuming a global
coordinate system and the body coordinate system are aligned, the
receiver 14 can measure three component values (x,y,z) of the
magnetic field H according to Equation 33, as follows:
.function..function..function..function..times. ##EQU00027## or for
a pure sine signal, according to Equation 34, as follows:
.function..times..function..omega..times..times..times..function..omega..-
times..times..times..function..omega..times..times..times..function..omega-
..times..times..times..function..omega..times..times..times..function..ome-
ga..times..times..function..times.e.times..times..omega..times..times..tim-
es.e.times..times..omega..times..times..times.e.times..times..omega..times-
..times..times. ##EQU00028##
The global and body coordinate system, however, are not necessarily
aligned. The relationship between these coordinate systems is
described by a 3.times.3 time-dependent rotation matrix Rot(t) so
that the receiver 14 actually measures H according to Equation 35,
as follows: {right arrow over (H)}.sub.Meas(t)=Rot(t){right arrow
over (H)}(t) (Eq. 35) where Rot(t) satisfies Equation 36, as
follows: Rot.sup.T(t)=Rot.sup.1(t) (Eq. 36)
It is important to realize that the square of the magnetic vector
is independent of the orientation of the receiver 14, as shown by
Equation 37, below: {right arrow over (H)}.sub.Meas.sup.T(t){right
arrow over (H)}.sub.Meas(t)=({right arrow over
(H)}.sup.T(t)Rot.sup.T(t))(Rot(t){right arrow over (H)}(t))={right
arrow over (H)}.sup.T(t){right arrow over (H)}(t) (Eq.37)
Variables may be extracted from measurements of the square of the
amplitude of magnetic field (Eq. 37). Assuming that magnetic
beacons of two transmitters 12 are generating fields H1 and H2 at
the location of the receiver 14 that can be described as: {right
arrow over (H)}.sub.1={right arrow over
(H)}.sub.1.sup.Rcos(.omega..sub.1t)+{right arrow over
(H)}.sub.1.sup.Isin(.omega..sub.1t)=Re({dot over
(H)}.sub.1e.sup.j.omega..sup.1.sup.t) {right arrow over
(H)}.sub.2={right arrow over
(H)}.sub.2.sup.Rcos(.omega..sub.2t)+{right arrow over
(H)}.sub.2.sup.Isin(.omega..sub.2t)=Re({dot over
(H)}.sub.2e.sup.j.omega..sup.2.sup.t) (Eq.38) The output of a
receiver 14 exposed to magnetic field (Eq. 34) will still be
described by equations (36) and (37): {right arrow over
(H)}.sup.T(t){right arrow over (H)}(t)=({right arrow over
(H)}.sub.1.sup.Rcos(.omega..sub.1t)+{right arrow over
(H)}.sub.1.sup.Isin(.omega..sub.1t)+{right arrow over
(H)}.sub.2.sup.Rcos(.omega..sub.2t)+{right arrow over
(H)}.sub.2.sup.Isin(.omega..sub.2t)).sup.T({right arrow over
(H)}.sub.1.sup.Rcos(.omega..sub.1t)+{right arrow over
(H)}.sub.1.sup.Isin(.omega..sub.1t)+{right arrow over
(H)}.sub.2.sup.Rcos(.omega..sub.2t)+{right arrow over
(H)}.sub.2.sup.Isin(.omega..sub.2t))++Noise (Eq. 39) Combining the
frequency terms of Equation 39, using Equation 40 below, one
derives:
.function..function..times..function..times..times..omega..times..times..-
function..times..times..omega..times..times..function..times..times..omega-
..times..times..function..times..times..omega..times..times..function..ome-
ga..omega..times..times..function..omega..omega..times..times..function..o-
mega..omega..times..times..times..times. ##EQU00029##
Coherent detection at double beacon frequencies and the beat
frequencies will recover values of each of the terms in Equation
40. For example, using Equation 41 below, one can recover the fifth
and seventh terms of Equation 40:
.times..intg..times..times.d.function..function..function..omega..omega..-
times..times..times..times..intg..times..times.d.function..function..funct-
ion..omega..omega..times..times. ##EQU00030##
Equation 40 does not permit complete recovery of the vectors. Each
of the vectors has 3 components for both real and imaginary parts.
Thus, there are 12 unknown variables in Equation 40 and only 8
sub-equations. However, Equation 40 does permit recovery of a very
important value, namely, the cosine of the angle between vectors of
real parts of magnetic field generated by the two transmitters 12
(1 and 2):
.function..alpha..times. ##EQU00031## One can determine the
numerator of Equation 42 from Equation 40.
In an isotropic media, the denominator of Equation 42 can be
recovered as well. There are eight sub-equations and eight unknowns
in Equation 40, namely: |{right arrow over (H)}.sub.1.sup.R|,
|{right arrow over (H)}.sub.2.sup.R|, |{right arrow over
(H)}.sub.1.sup.I|, |{right arrow over (H)}.sub.1.sup.I|, |{right
arrow over (H)}.sub.1.sup.R{right arrow over (H)}.sub.2.sup.R|,
|{right arrow over (H)}.sub.1.sup.I{right arrow over
(H)}.sub.2.sup.I|, |{right arrow over (H)}.sub.1.sup.R{right arrow
over (H)}.sub.2.sup.I|, |{right arrow over (H)}.sub.2.sup.R{right
arrow over (H)}.sub.1.sup.I| In non-isotropic media, Equation 42
can be solved only approximately, but at low enough frequencies
with sufficient accuracies.
FIG. 7 provides an error analysis for the positioning system 10.
This analysis assumes that there is a +/-5.degree. error in the
measurement of the vector direction. Through integrating and signal
processing, this can be reduced to +/-1.degree.. However,
geological effects and the presence of anomalous secondary
radiators increase that uncertainty to approximately +/-5.degree..
Through the use of precision frequency control and external
synchronization of the transmitters 12 and receiver 14 through the
initial set-up data 23 and back channel or surface-to-subsurface
communications, it is possible to reduce this final uncertainty by
an additional factor.
The positioning system 10 can use potential distance, but
cooperative sources assist in reducing the depth uncertainty.
Higher power transmitters 12 can be used to excite a swept
frequency chirp or other multi-frequency signal. Due to the
frequency dependence of depth of penetration of electromagnetic
waves in the ground, the receiver 14 antenna 31 in the underground
is able to detect the increased attenuation of higher frequencies
within the chirped signal and thereby provide an additional
constraint of the depth of the receiver 14.
The positioning system 10 can have a short set up time, can be
easily operated by field personnel, and affords the ability to
deploy worldwide. The system 10 consists of rugged magnetic
transmitters 12 (beacons) operating in the very low/low frequency
range. The system 10 can be deliverable by air or manual means and
is unaffected by most nearby structures.
Deployment of transmitters 12 can be conducted in several ways. The
transmitters 12 may be air dropped by fixed-wing aircraft, rotary
aircraft or emplaced manually. An all terrain vehicle may be used
to place the transmitters 12 in the desired location providing the
optimum overlay pattern. The transmitters 12 should be placed in
such a manner that at least three of the signals 40, 40', 40''
overlap each other in the effective beacon range, as shown in FIG.
8. To ensure adequate coverage of the transmitter 12 beacon range,
signal emissions 40, 40', 40'' can form an umbrella over the target
area 50.
To initiate use of the positioning system 10, field personnel can
synchronize their receivers 14 with transmitters 12 verifying
connectivity by signal display on their receivers 14. Once each
transmitter 12 is placed and activated, they can turn on and auto
locate by using a Global Positioning System (GPS). Upon GPS lock,
the transmitter 12 can begin emitting location and orientation
signals to the receiver 14 (FIG. 1). Transmitter 12 locations and
orientation are sent to the receiver 14 prior to entering an
underground facility. The operator can ensure that the receiver 14
initializes with the transmitters 12 prior to going underground and
that track logging is operational. An operations center located off
site, but in proximity to the application site, may be established
to monitor the current position of the positioning system receivers
14 underground.
The positioning system 10 receiver 14 can be mounted on an
all-terrain vehicle or worn in a backpack. The receiver 14 can be
configured in a man pack mode or an ATV configuration. All
necessary accessories are compatible with either configuration. The
receiver 14 can display current grid location, bearing, path
tracking, critical waypoints of interest, and battery life. The
receiver 14 can be an operator controllable, backlit, drill down
menu based platform. The menus can be designed to be easily
navigated and user friendly.
The transmitters 12 and receivers 14 can have an active life cycle
of up to 30 or more continuous operating hours, extendable with
additional batteries. In the event field operations exceed the life
cycle, the batteries can be manually replaced or new transmitters
12 can be deployed. An internal memory battery 42 (FIG. 2) can
prevent data receiver 14 loss in the event of the primary battery
failure. To conserve beacon 12 battery 16 power and limit operating
signature, programmable time delay and wake up capability can be
used when transmitters are emplaced prior to operations.
A back channel communication link using disposable transceivers 36
(FIG. 3) or surface-to-subsurface, bidirectional communications
using magnetic dipoles 34 (FIG. 5b) can be used to communicate with
the surface transmitter/receiver and other operational elements.
These transceivers 36 can provide line of sight data relay along
the tunnels whereas magnetic dipoles 34 need not rely on such. The
individual transceivers 36 can form a sparse network capable of
relaying data between above ground and below ground units. The
receiver 14 can have the ability to send low data rate
communications to the above ground receiver. This can enable the
remote control center to track the location of the positioning
system receivers 14 underground and communicate with each receiver
14 operator.
Underground navigation and mapping can be conducted in multiple
ways. In the back-packed configuration, a single operator can
operate and carry the receiver 14 while exploring the underground
environment. With the receiver 14 mounted on a vehicle, the vehicle
operator can operate the positioning system 10 hands free while
data is sent to the surface receiver. The hand held receiver 14 is
attachable to the operator's equipment. The mobile control center
can have the same graphic representation of the mapping and
underground navigation as the underground operator.
Beyond geophysical exploration, other potential applications of the
positioning system 10 concept include remote surveying of abandoned
underground mines, natural cavern exploration and surveying, and
underground mine and cavern rescue or similar uses. Moreover, this
embodiment is not limited to underground applications, but can be
applied in a variety of environments, including above ground
locations. In particular, another embodiment will now be described
in detail.
In traditional geophysical surveying using electromagnetic
approaches, the presence of conductors near the source and receiver
14 can be minimized through careful collection planning. However in
the positioning system 10, operational sites may have surface
conductors near the locations where transmitters 12 are deployed.
These conductors may be in the form of pipes, tunnel lining, and
boreholes could be present throughout the area operated. The site
could also include underground conductors near the field of the
receiver 14. For navigational purposes, all of these are
problematic and represent a significant source of noise which may
impede the proper operation of the positioning system 10. The
preferred embodiments described herein can address all of these
functional elements: validation of theoretical models; development
of magnetic field templates to support the location algorithms; and
development of automated procedures for separating clutter from the
direct transmitted signals.
For the positioning aspects of this system, this natural and
man-made noise is a potential hindrance to the positioning system
10 performance. In another embodiment of this system 10 shown in
FIG. 9, the noise is actually a source of useful signal information
which can be analyzed to reveal significant or important
information about the material composition and/or hydrology of the
Earth surface 5 within the volume of influence of the positioning
system 10 transmitters 12. Several different means are possible to
alter the behavior and performance of the positioning system 10 to
conduct investigation of the geophysical properties of subsurface
materials.
VLF coherent magnetic scanning or strategic hardened facilities
(SHF) and underground facilities (SHUF) provides an observer using
the system 10 information on distribution of conductive materials
and magnetic materials underground. The receiver 14 is able to
distinguish a motor or generator from a stainless steel reactor or
large piece of communication equipment. The system 10 can detect
reinforced tunnels also. The system 10 can provide detailed
information on what is behind radio frequency shielding that ground
penetrating radar cannot. If the earth surface 5 is too conductive
for ground penetrating radar to be useful, this embodiment allows
detection of both reinforced and unreinforced tunnels.
The VLF coherent magnetic scanner is a combination of two or more
vehicles 101 and 102, as shown in FIG. 9. Multiple transmitters 12
in the extremely low/very low/low frequency ranges are employed as
the radio frequency magnetic field beacons. Depending on the
desired information and specific access availability, similar
transmitters 12 are also employed within the underground space and
in vertical and/or horizontal boreholes. For geophysical
applications, transmitters 12 can transmit either single frequency,
swept frequency, or some other signal mode to simultaneously
maximize location determination for receiver units 14 and provide
enhanced data to support geophysical interpretations. Transmitter
12 locations and orientations are passed by a radio frequency link
to a receiver 14 as set-up data 23 before the receiver 14 goes
underground. The underground receiver unit 14 again comprises of a
three-component receiver to detect the transmitters 12, other
extremely low/very low/low frequency sources, and similar signals.
The underground receiving unit 14 can also be employed above ground
and/or in vertical or horizontal boreholes to enhance geophysical
signature collections. Additional geophysical sensors can be
deployed simultaneously to aid in the interpretation.
The two or more vehicles (e.g., remote controlled drones or surface
vehicles) 101 and 102 carry a magnetic transmitter 12 and a
receiver 14. A transmitter 12 is mounted on a first drone 101 and a
receiver 14 is mounted on a second drone 102. The receiver 14
measures magnetic field values over a large area and attempts to
measure the equivalent values of the induced fields 103, 104
generated by underground objects in the site of interest. The
induced fields are related to the volume of magnetically active
materials and thus the size and positions of underground objects
105, 106. The vehicles traverse the space above the site of
interest intended to be scanned. The transmitter 12 generates a
magnetic dipole reference field with an extremely stable frequency,
e.g., synchronized to the GPS and well characterized magnetic field
distribution. The receiver 14 measures in-phase and quadrature
values of all three components of the magnetic field. All
measurements are performed at frequencies around 1 kHz. The
measurements are solved to determine the distribution of equivalent
magnetic sources underground. The in-phase sources correspond to
magnetic materials, e.g., motor generators. The quadrature sources
correspond to conductive materials, such as aluminum structures,
cables, etc.
This embodiment defeats conventional shielding techniques. The 1
kHz frequency makes the system relatively insensitive to poorly
conductive elements such as reinforced concrete, minerals with high
water content, etc. Conventional shielding techniques such as 1/16
inch thick copper sheet will not prevent probing using system 10
with very low frequency as described above. A user of the system 10
in this manner may increase the sensitivity to the conductive
materials by increasing the frequency. Conversely, the user may
decrease the frequency to decrease sensitivity to the environment.
Use of primary frequencies below 10 kHz also minimizes potential
interference from naturally occurring sources such as distant
lightening storms which produce reduced noise levels in this
frequency range.
This technique is different from geological magnetic sounding
because it does not attempt to measure distribution of magnetic
properties of subsurface materials. At very low frequencies primary
and secondary magnetic fields may easily be separated. Induced eddy
currents are orthogonal (in quadrature) to the magnetic field.
Thus, the secondary magnetic field that they generate is in
quadrature to the primary field. If the latter is small and
tertiary effects can be neglected (i.e., low frequency or low
conductivity), the in-phase and quadrature field measurements can
separate primary and secondary magnetic fields, and thus,
perturbations caused by ferromagnetic materials, e.g., steels, and
conductive materials, e.g., copper and aluminum, can be
detected.
The processes and devices described above illustrate preferred
methods and typical devices of many that could be used and
produced. The above description and drawings illustrate
embodiments, which achieve the objects, features, and advantages of
the present invention. However, it is not intended that the present
invention be strictly limited to the above-described and
illustrated embodiments. Any modifications, though presently
unforeseeable, of the present invention that come within the spirit
and scope of the following claims should be considered part of the
present invention.
* * * * *